Good bike fit is critical. It is a miserable experience having backache, wrist ache and a numb posterior knowing there are three hours to go. Spend a good few hours experimenting with subtly different saddle and bar positions. Even a couple of mm in fore / aft or up / down adjustment can make a massive difference. Such experimentation is best done on a long solo ride where no one else is going to be pissed off with you stopping every 15 mins to faff about. Thing is, the faffing pays off once you find that sweet spot. Long easy training rides to acclimatise yourself to sitting all day in the roadie position are beneficial.
On the big ride, never be tempted to push on unless carrying speed up the next rise. A lower gear / higher cadence conserves energy towards an ideal of max momentum from min effort.
Wear the same gear you wore on the last couple of training rides, nothing should be untested.
As said above, fuel the engine with real food. Know in advance where to buy cake, nuts, chocolate milk. Buy most and carry a little, a tenner weighs nothing. Use a small saddlebag for tools, spares, snacks, waterproof jacket, to keep weight off your back and behind.
Break the ride down into a few natural sections, hill crests, junctions, cafe stops. Bigger gaps between stops when fresh, more frequent towards the end. Don’t stop too often. Drink lots of water, enjoy the challenge.